Shaped calcium silicate products are widely used, particularly as insulation materials in the building industry. These products are commonly formed by combining a source of calcium oxide with a siliceous material. On heating this mixture in an autoclave, a variety of crystalline forms of calcium silicate may be formed, depending upon the temperature, pressure, length of reaction time, and water concentration used. The reaction product of this process is generally an aqueous slurry of hydrated calcium silicate crystals. Reinforcing fibers such as glass and polyester fibers may then be added to the slurry. This slurry is then cast into molds and is dried, usually by the application of heat, to form the desired finished shaped objects.
It has been considered impractical by those in the industry to utilize any source of silica in the reaction which was not of extremely high purity. This is because it was believed and found that if a non-high purity source of silica was used, for some reason the calcium silicate hydrothermal reaction was greatly accelerated and the resulting reaction slurry was not easily moldable into shaped products because of poor filtration characteristics and poor calcium silicate crystal agglomerate formation.
It would be desirable, however, to be able to utilize non-high purity sources of silica in the reaction because of their ready availability and relatively cheap cost.